1. Field of the Invention
The present invention relates to a solid state image sensing device including pixels that produce electric signals corresponding to incident light. In particular, the present invention relates to a solid state image sensing device including pixels that are made up of transistors.
2. Description of Related Art
Solid state image sensing devices that are used for various applications can be classified under two groups, which are a CCD type and a CMOS type, in accordance with a difference of means for reading (extracting) photocharge generated in photoelectric conversion elements. The CCD type stores photocharge in potential wells and transfers the photocharge using the same, so a dynamic range of this type is narrow, which is a disadvantage thereof. On the other hand, the CMOS type reads photocharge stored in a pn junction capacitance of a photodiode through a MOS transistor directly.
In addition, one of conventional CMOS type solid state image sensing devices performs a logarithmic transformation operation in which quantity of incident light is processed by logarithmic transformation (see JP-A-11-313257). This solid state image sensing device has a wide dynamic range of 5-6 digits. Therefore, even if a subject of image sensing has a luminance distribution of a little wide luminance range, the image sensing device can convert entire luminance information within the luminance distribution into an electric signal to be produced. However, since an image sensible range is wider than the luminance distribution of a subject, there may be generated an area without luminance data at a low luminance area or a high luminance area within the image sensible range.
The applicant has disclosed a CMOS type solid state image sensing device that can switch between the above-mentioned logarithmic transformation operation and a linear transformation operation (see JP-A-2002-77733). In addition, for the purpose of performing the switch operation automatically between the linear transformation operation and the logarithmic transformation operation, the applicant has disclosed a CMOS type solid state image sensing device that sets a potential state of a transistor connected to a photodiode for performing a photoelectric conversion operation to an appropriate state (see JP-A-2002-300476). This solid state image sensing device disclosed in the JP-A-2002-300476 changes a potential state of a transistor so as to switch a point of inflection in which the photoelectric conversion operation is switched from the linear transformation operation to the logarithmic transformation operation.
In addition, there is disclosed another conventional solid state image sensing device that includes a pixel having a floating node as shown in FIG. 8 (see JP-A-2002-051263). This solid state image sensing device includes a photodiode PD that works as a photosensitive element, a MOS transistor T1 having a source connected to an anode of the photodiode PD, a MOS transistor T2 having a source connected to a drain of the MOS transistor T1, a MOS transistor T3 having a gate connected to a connection node between a drain of the MOS transistor T1 and a source of the MOS transistor T2, and a MOS transistor T4 having a drain connected to a source of the MOS transistor T3.
A dc voltage VPS is applied to a cathode of the photodiode PD and back gates of the MOS transistors T1-T4, and dc voltages VRS and VPD are applied to drains of the MOS transistors T2 and T3, respectively. In addition, signals φTX, φRS and φV are supplied to gates of the MOS transistors T1, T2 and T4, respectively, and a source of the MOS transistor T4 is connected to an output signal line 14. The MOS transistors T1-T4 are N-channel MOS transistors.
As shown in FIG. 10, when light enters the photodiode PD, photocharge is generated so that potential of the photodiode PD is decreased in accordance with the generated photocharge (the upper the position is, the lower the potential is in FIG. 10). In this case, a potential generated in the photodiode PD becomes a value that is proportional to an integral value of the quantity of incident light in a linear manner. Then, a voltage due to the potential of the photodiode PD is transferred via a transfer gate TG to an N type floating diffusion layer FD, and an electric signal of the transferred voltage is output as the image signal.
This solid state image sensing device outputs a noise signal and an image signal after reset in series when the signals φV, φRS and φTX are given at timings as shown in FIG. 11A. More specifically, the signal φRS is set to the high level so that the N type floating diffusion layer FD is reset to a potential of the dc voltage VRS. Then, the signal φRS is set to the low level, and a pulse signal φV that becomes the high level is given, so that the noise signal corresponding to a reset voltage is output. Then, the signal φTX is set to the high level, and the photocharge stored in the embedded photodiode PD is transferred to the N type floating diffusion layer FD. After that, the signal φTX is set to the low level, and the pulse signal φV that becomes the high level is given, so that the image signal corresponding to incident light is output.
The image signal and the noise signal obtained as described above are given to a subtracting circuit, which subtracts the image signal from the noise signal so that an image signal without a noise is obtained. However, if a subject of image sensing has a high luminance, charge that is overflowed from the photodiode PD during a time period t1 from the time of resetting the signal φRS to the low level to the time of setting the signal φV to the high level may flow in the N type floating diffusion layer FD. As a result, a potential of the N type floating diffusion layer FD is decreased. Therefore, a voltage of the noise signal becomes higher than a voltage of the image signal normally, but a voltage of the noise signal becomes lower than a voltage of the image signal, so the image signal obtained from a differential amplifier is decreased. Thus, reversed pixels may be generated in the high luminance area.
On the contrary, there is another device that prevents the overflow of photocharge from the photodiode PD by maintaining the signal φRS at the high level during the period until giving the pulse signal φV for output of the noise signal as the timing shown in FIG. 11B. However, even if this timing is used for driving, quantity of photocharge that flows in from the photodiode PD becomes larger than resetting the N type floating diffusion layer FD by the dc voltage VRS that becomes the reset voltage when a subject of image sensing has a very high luminance. Thus, a potential of the N type floating diffusion layer FD is decreased. As a result, reversed pixels may be generated in the very high luminance area.
A state of the signal when the reversed pixels are generated is shown in FIG. 12. In accordance with a horizontal synchronizing signal, a noise signal and an image signal of each row is output. A voltage of the noise signal is lowered in the reversed pixel, and a voltage level thereof varies substantially. In such a pixel that outputs a noise signal with a voltage level varying substantially, a noise signal becomes larger than an image signal. Therefore, an image signal of the reversed pixel is output from the differential amplifier. This problem of reversed pixels may be conspicuous particularly in a solid state image sensing device that performs the logarithmic transformation operation described above because there is increased chance of image sensing of a subject having a very high luminance area.